1. Field of The Invention
The invention relates to optical scanning systems and to the correction of video signals derived from scanners in such systems. In particular, it relates to the correction of video signals to provide new, or processed, video signals which more accurately represent objects scanned by an optical scanner.
2. Description of The Prior Art
Raw or unprocessed video signals produced by a scanner in an optical reading system deviate from values which would accurately represent characters, or other information, being read. The deviations are due to a number of variables in the system, such as the following: (1) Illumination is not uniform across the field being scanned due to such things as differences in the light produced by a number of light sources, where the differences stem from differences in aging or differences which were built-in when they were manufactured. (2) Sensors in the scanner vary in sensitivity producing non-uniform output values from cell-to-cell. (3) Optical characteristics in the system vary due to such things as cos.sup.4 law effects and dust. (4) Reflections from the documents vary due to such things as differences in paper and differences in surface coatings.
Among the known prior art apparatus designed to overcome the variations in video signals are that disclosed in a U.S. patent application No. 35,993, now abandoned, filed in the name of Pepe Siy, Robert Bloss and Gary B. Copenhaver, on May 4, 1979 entitled "Adaptive OCR Front-End System" which was assigned to the same assignee as the present application. That prior art application discloses use of an equalization filter to compensate for differences in illumination detected over the face of a document and provides an equalized video output. The equalized video is stored and used for comparison with subsequent similarly attained equalized video to produce a corrected video. The present invention is patentably distinct in that it affords an improved system which provides corrections for variations in individual pixels caused by a wide range of parameters such as lack of uniformity in illumination, variations from cell-to-cell in a sensor array, optical effects such as cos.sup.4 law and dust, and differences in reflectances in different documents.
The present invention is useful as part of a system for reading and processing data in various formats such as that disclosed in a copending U.S. patent application Ser. No. 301,117, now abandoned, in the names of Pepe Siy and James Young entitled "Geometric Character Recognition Using Skelton and Stroke Width Representation" and assigned to the same assignee as the present invention. An example of a Thinner and Stroke Width Analyzer in combination with Feature Extraction is shown in U.S. Pat. No. 4,162,482 entitled "Preprocessing and Feature Extraction System for Character Recognition" which issued July 24, 1979 in the name of Chauchang Su and was assigned to the same assignee as the present application. The foregoing applications and patents are hereby incorporated by reference.